Plasminogen activator activity and inhibition in rabbit tears after photorefractive keratectomy

Plasminogen activator is a normal component of tear fluid that plays a role in corneal wound healing processes. This work examines whether inhibitor-induced low levels of plasminogen activator activity (PAA) during corneal re-epithelialization after excimer laser photorefractive keratectomy (PRK) correlates with the eventual occurrence of haze in rabbit eyes. Tear samples were collected with glass capillaries from 16 eyes of eight New Zealand rabbits, using i.m. injection of pilocarpine hydrochloride for stimulation. Tears were collected before and after PRK surgery, and then daily for 5 days, and every fourth day thereafter for 3 months. Both eyes underwent PRK treatment. One eye of each rabbit was treated as a control while the contralateral eye was treated with aprotinin, a serine protease inhibitor, over the first 7 days. PAA in the tear samples was measured by a spectrophotometric method using human plasminogen and chromogenic peptide substrate S-2251. For the eight control eyes after PRK, the PAA values were significantly lower (day 1) and higher (days 2 and 3) than the equilibrium PAA (p<0.001). The corneas remained clear in each of these control eyes. For the eight contralateral aprotinin-treated eyes after PRK, the PAA values on days 1-7 were significantly lower than the equilibrium PAA (p<0.001). All eight of these aprotinin-treated eyes developed corneal haze after 2 months. There was no significant difference (p=0.06) between control and aprotinin-treated eyes for the equilibrium PAA after 19 days. We conclude that a corneal wound healing abnormality (haze) develops in rabbit eyes after PRK when PAA levels are reduced using aprotinin for a week following PRK.